Fix build warnings

[fpj]

Problem description
There are a number of warnings that need to be fixed, I'm posting them in a txt file attached.

Problem location
Build warnings.

Suggestions for an improvement
Fix the code to eliminate the warnings.

flink-connector-warnings.txt

[crazyzhou]

There is an issue #237 for this, I also pay attention to this along the implementation. I have already taken effort to eliminate some in the 1.10 upgrade and more (mainly in Table API methods) is done in this PR #363 , but there are still one thing we can't easily upgrade that is the BatchTableSource and BatchTableSink. Here are some reasons.

1. These APIs are deprecated because the Blink planner is unifying the Batch into the Stream. The suggested change is to use InputFormatTableSource and OutputFormatTableSink which is an extention of StreamTableSource/Sink and still marked experimental.
2. I cannot directly change to use the new API, because we rely on the batch table factory https://github.com/pravega/flink-connectors/blob/master/src/main/java/io/pravega/connectors/flink/FlinkPravegaBatchTableSourceFactory.java#L38 which needs to give out a BatchTableSource/Sink. There is another issue Change to Blink planner implementation #356 for this change entirely.
3. These APIs are still used in the Flink code. https://github.com/apache/flink/blob/master/flink-table/flink-table-api-java-bridge/src/main/java/org/apache/flink/table/sinks/CsvTableSink.java#L41
4. As a bigger picture, I'm not going to rush on Change to Blink planner implementation #356 because the whole FLIP-32 is still in progress. And soon, it will have a new structure of the table API. (ref. FLIP-95). It will be better to wait the final interfaces ready on Flink side first.